The present invention relates to a hoisting device, provided with a mast, on the top side provided with cable blocks; a trolley, which is movably fixed on the mast, on the top side is provided with cable blocks; and on the bottom side is provided with means for gripping a load; hoisting means, at least equipped with a hoisting cable and a winch, said hoisting cable being guided over the cable blocks of both the mast and the trolley, and it being possible to move the trolley relative the mast with the aid of the hoisting means; and a compensator, in the form of a pneumatic or hydraulic cylinder, for damping movements of the vessel as a result of heave and beating of the waves.
Various hoisting devices are known from the prior art. These hoisting devices are used in the offshore industry as drilling masts on, for example, drilling vessels. When, in use, a drill string is attached to the bottom side of a trolley, also known as a traveling block, the compensator has to compensate for the movements of the vessel relative to the seabed. The drill string itself will rest at least partially in the earth""s surface during the drilling and will make a minimal movement relative to the earth""s surface. The vessel, on the other hand, does move under the influence of the waves and the flow of the water.
According to the prior art, the compensator is generally placed between two blocks or trolleys, both of which can move relative to the mast. In this case the top trolley will be provided with cable pulleys, which can be moved relative to the mast with the aid of a hoisting cable. The bottom trolley will be attached to the top trolley by means of the compensator. When in this construction forces are exerted by the drill string upon the bottom trolley, these forces will be transmitted only partially to the top trolley.
The compensator generally used operates hydro-pneumatically. The hydro-pneumatic compensator will therefore be connected to a compressed air device by means of hoses, pipes and the like. A relatively large stroke volume is necessary for good functioning of such a compensator. Since both blocks or trolleys move relative to the mast, the compensator will also be able to move relative to the mast, which is a disadvantage. The connections of the compressed air device to the compensator must in fact also be able to move relative to the mast. This requires the use of, for example, flexible hoses and pipes, and all that makes the connection relatively complex, and therefore expensive.
It is a first object of the present invention to provide for a hoisting device according to the type mentioned in the preamble, in which the connections of a compressed air device to the compensator can be fitted at a stationary point.
That object is achieved in the present invention by the fact that the hoisting cable is guided over cable pulleys which are connected to the end of the compensator, all the above in such a way that force can be exerted upon the hoisting cable with the aid of the compensator.
That means that the compensator is no longer placed between the trolleys which are attached to the mast, but that the compensator acts directly upon the hoisting cable. The compensator can be connected by a first end to a stationary section of the mast. At the other end, the compensator is connected to the hoisting cable by way of cable pulleys. Tension can thus also be applied to the hoisting cable by means of the compensator.
The advantage of these measures is in the first place that the compensator can be fastened in a fixed position in the vicinity of the mast. The connection of the compressed air device to the compensator can therefore be made a t one point. That makes a relatively simple and cheap construction possible.
The hoisting device according to the invention can be improved further by the device comprising at least two compensators, each of which is connected to cable pulleys at its end.
The effect of this measure is that the device acquires greater redundancy. If the compensator in a device according to the prior art breaks down, the drilling operations must be stopped immediately With a hoisting device according to the invention, containing more than one compensator, it is possible to continue working should one of the compensators break down. The cylinder of the compensator which fails is locked in that case. Locking the compensator will mean that the stroke of the bottom trolley is reduced, but because one or more compensators that are still active remain, the device does not have to be shut down.
It is advantageous according to the invention for the mast to be designed in the form of a tube or sleeve, and for the compensator(s) to be place in the mast.